Exhaust gases of vehicles using gasoline as a fuel include harmful components such as total hydrocarbons (THC), carbon monoxide (CO), and nitrogen oxides (NOx). As such, it is necessary to purify each harmful component using a catalyst by oxidizing THC into water and carbon dioxide, by oxidizing CO into carbon dioxide, and by reducing NOx to nitrogen.
As catalysts for treating such exhaust gases (hereinafter referred to as “exhaust gas purification catalysts”), three-way catalysts (TWC) enabling reduction-oxidation of CO, THC, and NOx are used. The three-way catalysts are typically mounted at an intermediate position of an exhaust pipe between an engine and a muffler in a converter type.
As such a three-way catalyst, a catalyst adapted to support a noble metal on a refractory oxide porous body having a wide specific surface area, for instance, an alumina porous body having a wide specific surface area, and to support this on either a substrate, for instance, a monolithic substrate made of a refractory ceramic or metal honeycomb structure, or refractory particles is known.
In this type of three-way catalysts, the noble metal functions to oxidize hydrocarbon in the exhaust gas into carbon dioxide and water, to oxidize carbon monoxide into carbon dioxide, and to reduce nitrogen oxide to nitrogen, and a ratio of air to fuel (air fuel ratio) is preferably held constant (at a theoretical air fuel ratio) in order to effectively produce catalysis for both of the reactions at the same time.
In internal combustion engines of, for instance, vehicles, the air fuel ratio is greatly changed depending on driving conditions such as acceleration, deceleration, low-speed driving, and high-speed driving. As such, the air fuel ratio (A/F) that varies according to operational conditions of the engine is constantly controlled using an oxygen sensor (zirconia). However, since it is difficult for the catalyst to sufficiently exert purification catalyst performance in the case of merely controlling the air fuel ratio (A/F) in this way, a function of controlling the air fuel ratio (A/F) is also required of a catalyst layer itself. Thus, for the purpose of preventing a decrease in purification performance of the catalyst, which is caused due to a change in the air fuel ratio, using a chemical action of the catalyst itself, a catalyst in which a promoter is added to a noble metal that is a catalyst active component is used.
As such a promoter, a promoter (called an “OSC material”) having an oxygen storage capacity (OSC) to release oxygen in a reduction atmosphere and to absorb oxygen in an oxidation atmosphere is known. For example, ceria (cerium oxide, CeO2) or ceria-zirconia composite oxide is known as the OSC material having the oxygen storage capacity.
Ceria (CeO2) has a characteristic that can desorb and absorb attached oxygen and lattice oxygen in the cerium oxide depending on a level of an oxygen partial pressure in the exhaust gas and widens a range (window) of the air fuel ratio capable of efficiently purifying CO, THC, and NOx. In other words, when the exhaust gas has reducibility, the cerium oxide desorbs the oxygen (CeO2→CeO2−x+(x/2)O2), and feeds the oxygen into the exhaust gas, thereby causing an oxidation reaction. On the other hand, when the exhaust gas has oxidizability, the cerium oxide reversely takes the oxygen in an oxygen defect (CeO2−x+(x/2)O2→CeO2), and reduces an oxygen concentration in the exhaust gas, thereby causing a reduction reaction. In this way, the cerium oxide fulfills a function as a buffer that decreases a change in the oxidizability and reducibility of the exhaust gas, and functions to maintain the purification performance of the catalyst.
Further, the ceria-zirconia composite oxide in which zirconia is dissolved in this ceria is added to many catalysts as the OSC material, because the oxygen storage capacity (OSC) thereof is better.
With regard to the three-way catalysts using the OSC material such as ceria or ceria-zirconia composite oxide, the following inventions have hitherto been disclosed.
For example, in JP H06-219721 A, a catalyst that uniformly contains metal particles in metal oxide particles and that contains any of Pt, Pd, Rh, and Au as a noble metal and CeO2 as a metal oxide is disclosed as a metal-metal oxide catalyst having a novel catalyst characteristic.
In JP 2011-140011 A, a CO oxidation catalyst that supports Pd on CeO2 support particles and is formed by heat treatment in an oxidizing atmosphere at a temperature ranging from 850 to 950° C. is disclosed as a CO oxidation catalyst capable of showing CO oxidation activity over a wide range of temperatures including a low temperature.
In JP H10-277394 A, a vehicle exhaust gas catalyst in which a) active aluminum oxide that is fine and stabilized, b) at least one fine oxygen storage component, c) additional cerium oxide, zirconium oxide, and barium oxide that have high dispersivity, and d) one catalytic film layer made of palladium as the only catalytic noble metal are formed on an inactive substrate is disclosed as a catalyst that has high conversion rates of hydrocarbon, carbon monoxide, and nitrogen oxide and excellent heat and aging resistance and contains only palladium.
In JP 2005-224792 A, as a three-way catalyst in which Pd is supported on a support material based on a composite oxide containing Al, Ce, Zr, Y, and La, a catalyst in which a ratio BA of the total number B of moles of Ce, Zr, Y, and La atoms to the number A of moles of Al atoms in the support material is 1/48 or more and 1/10 or less, and Pd has a part in a metal state and the balance in an oxide state is disclosed.
In JP 2010-521302, a three-way catalyst configured to apply strontium oxide or barium oxide to a surface of a catalyst layer made of aluminum oxide, cerium/zirconium mixed oxide catalytically activated by rhodium, and cerium/zirconium mixed oxide catalytically activated by with palladium is disclosed.
During which two- and four-wheeled vehicles are driving, an oxygen excess condition (lean burn condition) on which an oxidation reaction is effective and a fuel excess condition (rich burn condition) on which a reduction reaction is favorable repeatedly alternate with each other in response to driving conditions. For this reason, it is necessary for the catalyst for the exhaust gas to exert predetermined catalyst performance or higher under any of the oxygen excess condition (lean burn condition) and the fuel excess condition (rich burn condition). Especially, in the case of the two-wheeled vehicle, there is a tendency to raise the number of rotations of an engine to drive the vehicle under a fuel-rich atmosphere. As such, there is a need to exert the catalyst performance, especially, under a fuel-rich atmosphere having a high space velocity (SV).
In the existing three-way catalysts, platinum (Pt) and rhodium (Rh) among noble metals have been frequently used as the catalyst active components. However, since these noble metals are extremely expensive, there is a need to develop a palladium catalyst in which less expensive palladium (Pd) is used a lot.
However, when palladium (Pd) is used as the catalyst active component, this has a problem that purification rates of carbon monoxide (CO) and total hydrocarbons (THC) are lowered, especially, under a fuel-rich atmosphere having a high space velocity (SV).
Therefore, an object of the invention is to provide, with regard to a palladium catalyst in which palladium (Pd) is used as a catalyst active component, a new palladium catalyst capable of efficiently purifying carbon monoxide (CO) and total hydrocarbons (THC) under a fuel-rich atmosphere having a high space velocity (SV).